In recent years, various attempts have been made to improve the optical performances of progressive power lenses. In particular, attention is being given to a progressive power lens produced on the basis of aspherical design. This is intended to compensate for an error portion of a lens caused by spherical design by assuming a condition equivalent to an actual condition under which a user wears spectacles in each of which the lens is assembled, and calculating the dioptric power, astigmatism, prism, etc. of the lens by means of ray tracing.
A progressive refractive surface is originally formed by smoothly connecting spherical surfaces at distance and near portions, which are different in curvature, to each other within one surface, and therefore, it is naturally an aspherical surface. However, the wording "the aspherical design of a progressive power lens" used in this description means that even a point, having a specific curvature, of a progressive refractive surface, such as a distance optical center or near optical center is a aspherical surface.
A progressive power lens produced on the basis of such an aspherical design is disclosed in Japanese Patent Publication No. Tokko-hei 2-39768, which lens exhibits effects of reducing astigmatism and thinning the lens as compared with a lens produced on the basis of spherical design.
In the case of designing and producing a lens in accordance with the technique disclosed in Japanese Patent Publication No. Tokko-hei 2-39768, however, there occur several problems or insufficient points.
At first, Japanese Patent Publication No. Tokko-hei 2-39768 discloses the structure of only a region, in the vicinity of a main meridian line extending between distance and near portions, of the progressive power lens. The main meridian line of a progressive power lens is certainly as important as to be called a main convergence line; however, it is only a line. When acquiring viewing information, a human being takes a sight of a wide area other than the meridian line.
At second, since the dioptric power of a progressive power lens differs between different positions of the lens, an ideal additional aspherical surface amount added to an original progressive refractive surface must differ depending on a position of the lens. In accordance with the technique disclosed in Japanese Patent Publication No. Tokko-hei 2-39768, the additional aspherical surface amount differs between the distance and near portions disposed along the main meridian line; however, it is unclear what aspherical surface is set at other portions.
The region disposed along the main meridian line also includes a progressive portion in which a focal power is continuously changed, and it is theoretically required to give an additional aspherical surface amount to such a progressive portion. At the present day, however, any prior art intended to give an additional aspherical surface amount to the above progressive portion has been not disclosed.
The progressive surface of a progressive power lens is required to be configured such that refractive surfaces at all portions of the lens are optically continuous to each other within one surface. If a lens is configured such that although refractive surfaces at portions along the main meridian line form an optically continuous aspherical shape, the refractive surfaces at other portions not along the main meridian line do not form an optically continuous aspherical shape, it is useless to produce the lens on the basis of the aspherical design. As a method for forming the refractive surfaces at the portions not along the main meridian line into an optically continuous aspherical shape, there is known only a method of interpolating a curvature in the direction extending, perpendicular to the main meridian line, from each point of the optically continuous aspherical portion along the main meridian line. It is not regarded that such a method is able to form the portions not along the meridian line into an ideal aspherical shape.
The production of a custom-made progressive power lens for spectacles is required to simply form a progressive aspherical shape on the basis of the optimum aspherical design for achieving effects of reducing astigmatism in accordance with the user's recipe including the dioptric power of the lens and of thinning the lens.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a progressive power lens in which all of portions including a progressive portion are formed into an optimum aspherical shape on the basis of a simple lens design.